Insights Into Neural Stem Cells May Point Way to New Tx

Signal molecule TGF-beta regulates neural stem-cell development.

Action Points

Note that Drosophila-based experiments have identified TGF-beta as an important regulator of neuronal cell differentiation.

Be aware that temporally-acting genes as well as time-invariant "identity" genes interact to lead to the neuronal changes of development.

New insights into how molecular time signaling controls stem cells at different stages of brain development could have implications for the development of novel treatments for psychiatric and neurodegenerative diseases, researchers reported.

Researchers from Karolinska Institute, in Stockholm, discovered that TGF-beta acts as a switch-signal in temporal patterning of the vertebrate brain that executes the transition between early and late phases of neurogenesis.

The identification of TGF-beta as a temporal switch signal and regulator of neural progenitor potential provides a future framework to modulate temporal identity and potency of neural cells in stem-cell engineering, researcher Johan Ericson, PhD, and colleagues wrote in the journal Neuron, published online Nov 13.

"This is the first known signaling molecule that regulates the potential of neuronal stem cells," Ericson stated in a written press release.

Mechanism of Cell Diversity in the Brain Poorly Understood

Earlier work suggests that in the developing vertebrate central nervous system, neural progenitors undergo an initial period of neurogenesis followed by a late phase when non-neuronal glial cells become specified. During the period of neurogenesis, it is believed that specific types of neurons are sequentially produced during defined time windows, and in this process, progenitors become progressively restricted in their developmental potential by losing competence to generate early-born neurons.

But almost nothing is known about the mechanisms that execute the transition from one temporal identity state to another and whether such switch decisions are coupled to the regulation of neural stem-cell potency, Ericson and colleagues noted.

Temporal neuronal patterning contributes to the establishment of cellular diversity in all regions of the brain, but is particularly evident in the developing retina and cerebral cortex in which birth-order of neuronal subtypes is correlated to the organization of cells into topologically different layers.

"While transcriptional networks regulating temporal patterning and progenitor competence in the Drosophila nerve cord have been well-defined, only a small number of transcription factors that provide temporal identity or competence to neural progenitors have been characterized in the vertebrate CNS," Ericson and colleagues wrote. "It has therefore been difficult to define the presumptive regulatory relationship between sequentially acting transcriptional fate determinants in vertebrates, which is a requirement in order to understand how progenitor competence is regulated over time, and how temporal switch decisions are mechanistically implemented."

TGF-beta Regulates Neuronal Stem Cells

Extrinsic signals have been shown to influence the transition between early and late phases of neurogenesis in the cortex, and it is believed that young cortical progenitors either have inherent competence to produce late-born neurons or acquire such competence in response to late-acting cues.

"Similar to the developing neocortex, extrinsic signals have also been implicated in the sequential specification of motor neurons, serotonergic neurons and oligodendrocyte precursors in the ventral hindbrain," the researchers wrote. "These cell types are generated from progenitors that express the homeodomain protein Nkx2.2, and young Nkx2.2+ progenitors can prematurely generate late-born serotonergic neurons when the early motor neuron-identify gene Phox2b is genetically ablated."

In their newly published study, Ericson and colleagues showed that TGF-beta signaling executes the motor neuron (MN) to serotonergic neuron (5HTN) temporal fate switch in ventral hindbrain progenitors and that TGF-beta suppresses early-born neurons and induces late-born cell types in different regions of the central nervous system.

Their work also showed that young Nkx2.2+ hindbrain progenitors have inherent competence to produce late-born 5HTN and that the timing of TGF-beta2 activation and execution of MN to 5HTN switch is intrinsically programmed within the Nkx2.2+ temporal lineage downstream of Sonic hedgehog (Shh).

"TGF-betas 1-3 are broadly and dynamically expressed in the developing CNS, raising the possibility that TGF-beta signaling could influence temporal fate specification in many regions of the neural tube," the researchers wrote. "Similar to the hindbrain, we observed a progressive up-regulation of TGF-beta2 expression in ventral progenitors in the developing midbrain."

The investigations by Ericson and colleagues showed that TGF-beta signaling executes the transition between early and late phases of neurogenesis and concurrently constrains the potency of Nkx2.2+ progenitors in the developing hindbrain. They wrote that TGF-beta acts as a devoted temporal switch signal that is not required for the specification of late-born cell types, and could be integrated in other temporal gene regulatory networks in which distinct subtypes of neural progenies are sequentially specified.

The TGF-beta-regulated primary MN-to-5HTN switch was found to be temporally coupled to the subsequent 5HTN-to-oligodendrocyte precursor (OLP) switch, indicating that the temporal onset of TGF-beta signaling triggers progenitor-age progression, which, in turn, affects the overall lifespan of the Nkx2.2+ temporal lineage.

"Intriguingly, this implies that the lifespan of temporal differentiation lineages in the vertebrate CNS is dynamic and not fixed in time," the researchers wrote. "This concept is important both at a mechanistic level and in evolutionary terms as it provides a means to regulate tissue size by modulating the production-period of a given cell type without inflicting on other cell types produced by the lineage."

Data Suggest 'Hierarchical Dominance' Model

The data suggest a novel "hierarchical dominance" model of sequential fate specification in which transcription factors promoting MN- and 5HTN-fate co-exist in young progenitors but where the activity of Phox2b predominates over 5HTN-fate determinants at early developmental stages, the researchers noted, adding that this hierarchical dominance mechanism differs conceptually from "sequential competence state" models in Drosophila, "as young neuroblasts appear to lack competence to produce late-born cells types even when early identity genes are genetically eliminated or suppressed in response to premature expression of temporal switch factors."

The regulation of progenitor competence and potency in the ventral hindbrain show several important similarities to temporal patterning of the developing neocortex, the researchers wrote.

"In particular, like Nkx2.2 progenitors, young cortical progenitors have competence to produce late-born progenies in response to extrinsic signals active at late stages of cortical development. The sequential generation of cortical neurons can furthermore be recapitulated from individual cortical-like progenitors in vitro, implying that the activation of presumptive cortical signals is intrinsically programmed within cortical temporal differentiation lineages."

The research by Ericson and colleagues suggests that the activation of TGF-beta2 and execution of the MN-to-5HTN fate switch is intrinsically programmed within the Nkx2.2+ temporal lineage downstream of Shh. Phox2b is required to suppress TGF-beta2 expression at young progenitor stages, and forced expression of the early cortical identity genes Fezf2 or Ikaros is sufficient to extend the phase of early neurogenesis at the expense of late-born neurons in the cortex.

"It is therefore feasible that opposing regulatory interactions between early-acting intrinsic temporal determinants and late-acting extrinsic signals could reflect a common strategy for how temporal switch decisions are regulated in temporal patterning processes in the vertebrate brain," Ericson and colleagues wrote. "It is also notable that the temporal switch activity of TGF-beta is intimately associated with a loss of potency of Nkx2.2+ progenitors to produce MNs, identifying TGF-beta as a temporal regulator of neural stem-cell potency."

The researchers concluded that their identification of TGF-beta as a temporal switch signal and regulator of neural progenitor potential provides a "future framework to modulate temporal identity and potency of neural cells in stem-cell engineering."

"Considering the important implications of 5HTNs in neurological disease, psychiatric disorders, and spinal cord repair, our study provides proof-of-concept that extrinsic cues regulating temporal neurogenesis can be utilized to bypass early phases of neurogenesis and thereby facilitate effective production of late-born and clinically relevant neurons from stem cells," they wrote.

The study was funded by the Swedish Foundation for Strategic Research, The Knut and Alice Wallenberg Foundation, the Swedish Research Council and the Royal Swedish Academy of Sciences.

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